The trend towards integration of ever-increasing numbers of functions in an integrated circuit leads to smaller and smaller design geometries and higher and higher processing speeds. If clocked CMOS circuits are used, the current drain is highly clock-synchronous and pulsed. Hybrid circuits do not differ in this respect if the package contains, besides analog circuits, clocked circuits, particularly CMOS circuits, with steep current pulse edges. Examples of clocked circuits with high pulsed current drain are control units, microprocessors, and digital signal processors for various applications. With the progress in miniaturization and the increase in switching speeds associated therewith, the harmonic content of these pulsed supply and load currents increases. A consequence of this is the conducted electromagnetic waves which leave the integrated circuit on the supply and input/output (I/O) lines. This may result in mutual interference in interacting circuit arrays or in nonpermissible electromagnetic emissions which interfere with the operation of other electronic equipment.
A critical application for clocked circuits is in the automotive field, because no shielding of the printed circuit boards and the associated connecting leads is possible there on cost grounds, so that radio-frequency interference can easily propagate via an unshielded board assembly. The interference is then audible in the radio set, car telephone, etc. Particularly in this field of application, the discussion about electromagnetic compatibility (EMC) has, in recent years, led to a definition of emission limits which new systems must meet.
The attempt to prevent the interference from leaving the integrated circuit by installing chokes in the supply lines, easily results in critical fluctuations in the voltage supply of the integrated circuit; the supply voltage may even briefly drop below its minimum value. Moreover, the installation of chokes in the supply lines does not suppress the interference conducted by the I/O lines. Elimination of such interference requires separate measures on each of the existing I/O lines.
How the pulsed supply and load currents act via the supply and I/O lines of an unshielded board network is illustrated schematically in FIGS. 1 and 2 by the associated radio-frequency and low-frequency current components. For simplicity it will be assumed that the ohmic resistances of the networks shown are small compared with the effective reactances for the radio-frequency current components.
Interference in the case of which the radio-frequency current components are conducted via relatively large-area current loops, is particularly unpleasant since, the area around which he radio-frequency currents flow defines directly the magnetically emitting area and is thus, Proportional to the radiated electromagnetic energy. Primarily, the supply lines of the clock circuit stages are affected. But the output drivers, too, may form such current loops via their I/O lines, the distributed capacitances of the latter, and the input capacitances of the connected systems, such as a display, with current feedback taking place over the supply or ground connection lines. In the case of I/O lines, very large-area current loops may occur, which then become effective as magnetically emitting areas in a similar manner. But even if no currents are flowing, the I/O lines cause disturbances through the capacitive emission of the signal edges and undesired level discontinuities.
Aside from appropriate I/O-line and ground-line routing, the interference caused by the signal edges on the I/O lines can be reduced by slowing down the signal edges or by arranging that the switching event occurs only very rarely and irregularly. However, a slowdown of the signal edges of the output drivers will be only partly effective if rapid level fluctuations are still present on the I/O lines. These undesirable level fluctuations are caused by rapid level fluctuations on the supply or grounded sides of the respective I/O drivers, with the insufficient choking of the supply lines playing an essential part, as mentioned above. In any case, the level fluctuations on the I/O lines have the same effect as if I/O drivers with steep signal edges were present.
It is therefore, an object of the invention to solve the emission problem on the supply and I/O lines of an integrated circuit within an unshielded board network, by providing a low-cost, spurious-emission-reducing terminal configuration for the integrated circuit.